The most common route to (meth)acrylic acid is through gas phase catalytic oxidation of alkanes, alkenes, alkanols or alkenals containing 3 to 4 carbon atoms. The reaction products from the oxidation process are separated, and (meth)acrylic acid is purified in the recovery and purification section. The design of the recovery and purification section varies in process equipment and operating conditions, but, fundamentally it comprises extraction and distillation separations. In a generalized (meth)acrylic acid recovery and purification process, the effluent from the oxidation process is cooled in an absorber to remove the light components in the product effluent. Then, in the extraction column, (meth)acrylic acid is concentrated through removal of either water or acetic acid or both with a selected solvent. The crude (meth)acrylic acid stream is then purified of remaining extraction solvent and reaction by-products in succeeding distillation towers. Design variations exist with solvent selection.
(Meth)acrylic acids are reactive monomers, and they tend to polymerize with any trivial environment change. This is the case during the recovery and purification operation in manufacturing (meth)acrylic acid, where elevated temperatures accelerate the polymerization of (meth)acrylic acid. Under such circumstances, the undesired polymerization becomes so severe that polymer deposition fouls process equipment. Eventually, equipment shutdown and cleaning is required for removal of the polymeric foulant. Conventionally, polymerization inhibitors are used in the manufacturing processes of the monomers to prevent this undesired polymerization. Typical polymerization inhibitors are phenolic compounds, amines, quinones, nitroxyl compounds and certain inorganic complexes. Phenothiazine (PTZ), hydroquinone (HQ) and monomethyl hydroquinone ether (MEHQ) are examples of the most widely used inhibitors. These inhibitors are designed to interrupt the polymerization reactions and prevent the formation of the polymer. However, none of the available polymerization inhibitors are efficient enough to completely eliminate undesired polymer formation. Even in the presence of these inhibitors, polymer formation and subsequent fouling is still substantial, so that periodic cleaning is part of routine (meth)acrylic acid processes.
In industrial practice, dispersants may also be used in addition to polymerization inhibitors to improve fouling prevention. Dispersants are usually comprised of molecules with an affinity to the foulant particle surface and good solubility in the liquid process stream. Unlike inhibitors, dispersants do not interfere with the polymerization reactions involved in foulant formation. Instead, dispersant molecules adsorb on preexisting polymer particles, through chemical or physical interaction, and form an insulating layer on the polymer particles, which prevent the particles from agglomerating, thereby keeping them suspended in the process media. The selection of an effective dispersant remains as an experimental art because their utility is highly dependant on the detailed nature of the foulant material and liquid medium, both of which are unique to a given process.